A Unique Recombinant Fluoroprobe Targeting Activated Platelets Allows In Vivo Detection of Arterial Thrombosis and Pulmonary Embolism Using a Novel Three-Dimensional Fluorescence Emission Computed Tomography (FLECT) Technology

Progress in pharmaceutical development is highly-dependent on preclinical in vivo animal studies. Small animal imaging is invaluable for the identification of new Disease markers and the evaluation of drug efficacy. Here, we report for the first time the use of a three-dimensional fluorescence bioimager called FLuorescence Emission Computed Tomography (FLECT) for the detection of a novel recombinant fluoroprobe that is safe, easily prepared on a large scale and stably stored prior to scan. This novel fluoroprobe (Targ-Cy7) comprises a single-chain antibody-fragment (scFv_Targ), which binds exclusively to activated-platelets, conjugated to a near-infrared (NIR) dye, Cy7, for detection. Upon mouse carotid artery injury, the injected fluoroprobe circulates and binds within the platelet-rich thrombus. This specific in vivo binding of the fluoroprobe to the thrombus, compared to its non-targeting control-fluoroprobe, is detected by the FLECT imager. The analyzed FLECT image quantifies the NIR signal and localizes it to the site of vascular injury. The detected fluorescence is further verified using a two-dimensional IVIS^® Lumina scanner, where significant NIR fluorescence is detected in vivo at the thrombotic site, and ex vivo, at the injured carotid artery. Furthermore, fluorescence levels in various organs have also been quantified for biodistribution, with the highest fluoroprobe uptake shown to be in the injured artery. Subsequently, this live animal imaging technique is successfully employed to monitor the response of the induced thrombus to treatment over time. This demonstrates the potential of using longitudinal FLECT scanning to examine the efficacy of candidate drugs in preclinical settings. Besides intravascular thrombosis, we have shown that this non-invasive FLECT-imaging can also detect in vivo pulmonary embolism. Overall, this report describes a novel fluorescence-based preclinical imaging modality that uses an easy-to-prepare and non-radioactive recombinant fluoroprobe. This represents a unique tool to study mechanisms of thromboembolic diseases and it will strongly facilitate the in vivo testing of antithrombotic drugs. Furthermore, the non-radiation nature, low-cost, high sensitivity, and the rapid advancement of optical scanning technologies make this fluorescence imaging an attractive development for future clinical applications.